Despite recent advancements in cellular mobile network architectures, full coverage for seamless handoffs is still an open issue. Moreover, cost efficient calls for the end users in urban regions with very high coverage can be improved. In many areas overlapping networks like WLAN, GSM and 3G provide alternative wireless links. Ideally, the user should be connected to the most attractive network any time according to predefined priorities using an automatic handover. In this paper, we present a framework for vertical handoff with dual- mode (cellular/WLAN) devices by using the conference bridging functionality of a PBX gateway. The communication between client and server will be realized via an overlay signaling channel. Based on the work done in we extend and integrate some important services in the proposed framework: The ICAS (Individual Communication Address Space) feature allows users to be reachable by an alias name without revealing his real phone number. Our proposed Presence Enabled Phonebook shows the system presence and status information of the users' contacts. Beyond the illustration of the framework, the quality and reliability of voice and overlay channel were analyzed in terms of a vertical handoff.